A transistorized high-voltage switch. In particular, a preferred embodiment of the present invention replaces conventional spark gap circuits used in a fireset incorporating an exploding foil initiator (EFI).
In high voltage switching circuits incorporating an EFI, for example, conduction may be initiated through a dielectric by the complete dielectric breakdown between electrodes separated by the dielectric, e.g., air. Closing a switch allows energy to pass from a high-voltage power supply across the electrodes to the EFI. Switches with high voltage ratings, i.e., kilovolts (kV), are needed to hold off the voltages used with an energy storage capacitor, i.e., 1-3 kV, for a single EFI. When triggered, such switches should produce a pulse having a fast rise time in order to properly initiate the EFI. Typical pulses have stored energies of 0.1-0.6 milli-Joules (mJ), rise times of 30-60 nanoseconds (ns), peak currents of 1-7 kiloamps (kA), and peak powers of 1-15 megawatts (MW).
A typical spark gap device incorporates an ionizable gas in a chamber separating two electrodes. Spark gap devices are usually coupled in parallel with a circuit or device to be used only upon a deliberate initiation, e.g., an EFI. When the electrical potential across the load and across the spark gap reaches the breakdown voltage for the ionizable gas, the gas ionizes, and current flow is established between the electrodes. For applications in which spark gap switches have been used in over voltage and over current protection circuits, this breakdown shunts the high potential around the circuit or device being protected. For applications in which the intent is to deliver a high voltage that has been xe2x80x9cheld offxe2x80x9d an initiator such as an EFI, the current is used to discharge a capacitor, thus delivering the necessary initiation current to the EFI.
Spark gap devices have been used to switch currents to other devices, as known from U.S. Pat. No. 3,275,891 issued to Swanson, September 1966. Such triggered spark gaps work well when the switching application requires rapid switch closure, but do not work well when rapid opening of the switch is required, as in the case of an over voltage protection circuit. Triggered spark gaps do not allow rapid and successive application of high voltage because the ionized gas in a triggered spark gap does not de-ionize rapidly. De-ionization is slow because the arc is surrounded by gas that does not allow the arc to cool rapidly. Using an electric field to activate a particular kind of switch is disclosed in U.S. Pat. No. 3,492,532 issued to Fayling, January 1970. An ionized flow path in a spark gap device is used to displace a liquid magnetic and electrically conductive material into a position inside the spark gap device which physically connects the electrodes, thus replacing the ionized gas flow path with a direct connection. A recent application of spark gaps in firesets includes a xe2x80x9cmicrogapxe2x80x9d in series with a high-voltage two electrode spark gap as disclosed in U.S. Pat. No. 5,641,935, Electronic Switching for Triggering Firing of Munitions, issued to Hunter et al, Jun. 24, 1997.
Alternatives to conventional spark gap switches have been proposed such as a device that requires moving parts as disclosed in U.S. Pat. No. 5,854,732, High Voltage Arcing Switch Initiated by a Disruption of the Electric Field, issued to Murray, Dec. 29, 1998. Although this switch is capable of handling voltages to 25 kV, it also requires a large package and because of the requirement for moving parts, it does not have an outstanding response time. Another alternative is the electrical safe arm device as disclosed in U.S. Pat. No. 5,436,791, Perforating Gun Using an Electrical Safe Arm Device and a Capacitor Exploding Foil Initiator Device, issued to Turano et al, Jul. 25, 1995 or U.S. Pat. No. 5,444,598, Capacitor Exploding Foil Initiator Device, issued to Aresco, Aug. 22, 1995. Although effective, these devices require complicated circuitry, a high voltage source on the order of 3 kV, and are not capable of outstanding response times.
Although transistors have been used in circuits for firing a detonator, these circuits have generally been low-voltage spark gap circuits, an example of which is U.S. Pat. No. 4,296,688, Electronic Circuit for Firing a Detonator, issued to Orlandi, Oct. 27, 1981. Further, the response time for these circuits is on the order of hundreds of milliseconds. Another example that seeks to avoid the bulk of a typical spark gap circuit is disclosed in U.S. Pat. No. 4,559,875, High Energy Switching Circuit for Initiator Means of the Like and Method Therefor, issued to Marshall, Dec. 24, 1985. This switch is intended for one-time use and consists of a number of junction diodes in series, providing the necessary reverse standoff voltage for a high-voltage source on the order of 3000 V. Yet another recent xe2x80x9cone-shotxe2x80x9d switch is disclosed in U.S. Pat. No. 5,249,095, Laser Initiated Dielectric Breakdown Switch, issued to Hunter, Sep. 28, 1993, wherein light from a laser source, possibly a laser diode, shines on dielectric material between two electrodes and initiates break down.
A commonly used spark gap switch is the ceramic-bodied, hard-brazed, miniature spark gap, incorporating either a vacuum or an ionizable-gas filled volume. Spark gaps require hermetic sealing, are expensive, have marginal reliability, a short operating life, and require expensive high-voltage trigger circuits. One other switch in use for this application is the explosively initiated shock conduction switch that uses a primary explosive detonator. This switch presents handling problems, producing chemical contamination and possible impact damage to nearby electronics.
Putting a high-voltage transistor in series with a component that has known electrical potential breakdown characteristics provides a simpler, less expensive, more durable and reliable circuit capable of repeated firings with outstanding response times and repeatability.
A preferred embodiment of the present invention envisions a dual-tube fireset incorporating two gas tube surge arrestors as part of a high voltage switch for switching a voltage on the order of 1000 V. Until a high-voltage Field Effect Transistor (FET) is energized, operating voltage is held off by the series combination of the surge arrestors and high-voltage resistors. Upon energizing the FET via a fire signal enabling a 28 V source, the voltage across the lower surge arrestor decreases and the voltage across the upper surge arrestor increases. Upon reaching the breakdown voltage in the upper surge arrestor, the gas inside the tube ionizes, becomes electrically conductive, and, upon breaking down, dumps its voltage across the lower surge arrestor. This causes the lower surge arrestor to also break down. Since both surge arrestors are now conducting and the 1000 V source is free to energize the circuit, the capacitor is discharged through the EFI. The breakdown of both arrestors occurs in nanoseconds. A high resistance bleed resistor is connected in parallel with the discharged capacitor. It will be used to bleed off the capacitor""s charge in the event that the EFI is not to be initiated.
In a preferred embodiment representing a fireset incorporating an EFI, the 28 V source supplies energy for discharging a capacitor into the gate of the FET, initiated by a 5 V firing signal and limited by a resistor in series with the gate of the FET. Switching is enabled in less than 0.1 xcexcsec, with excellent repeatability, durability, and reliability.
Advantages of preferred embodiments of the present invention as compared to switching and controls in existing firesets, in particular firesets employing spark-gap devices, include:
much faster response times, i.e., small delay and rise times for the electrical pulse;
long life over a wide operating temperature range, xe2x88x9254xc2x0 C. (xe2x88x9265xc2x0 F.)-88xc2x0 C. (190xc2x0 F.);
reduced size, providing inherent advantages for hermetic sealing of the circuit;
fewer and smaller components able to be purchased as COTS items;
reduced system capital costs;
the current to fire the EFI does not transit the FET;
increased operational readiness;
low maintenance costs;
high durability;
increased flexibility;
high reliability;
particularly suitable for modification and upgrade; and
readily applied to existing systems.
Embodiments of the present invention can be applied to any situation in which reliable repeated firing of an initiator is desired. Incorporating the invention into a design saves capital as well as operations and maintenance costs.
Preferred embodiments are fully disclosed below, albeit without placing limitations thereon.